Fluid feed system



May 13, 1969 H. J. KING 3,443,383

FLUID FEED SYSTEM Filed Dec. 19, 1966 Sheet r 2 Pan/2 Jup/u y i zal.

Array-9.

May 13, 1969 H. J. KIN-G 3,443,383

FLUID FEED SYSTEM Filed Dec. 19, 1966 Sheet 13 of 2 United States Patent U.S. Cl. 60-202 2 Claims ABSTRACT OF THE DISCLOSURE The disclosed invention relates to devices for discharging a fluid propellent to an ion thrustor and comprise a fluid reservoir, a piston in the reservoir and means for applying a constant force on said piston for expelling fluid at a constant pressure to the ion thrustor.

The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of section 305 of the National Aeronautics and Space Act of 1958, Public Law 85-568 (72 Stat. 435; 42 U.S.C. 2457).

This invention relates primarily to a fluid feed system and more particularly to methods and apparatus for controllably feeding propellant from a reservoir to an ion engine.

Prior to the present invention, liquid mercury propellant was fed from a mercury reservoir to a mercury ion thrustor by using a high pressure gas from a high pressure gas bottle whose pressure was controlled by a mechanical regulator in the feed line between the bottle and the reservoir.

The reservoir was a spherically shaped container with a flexible metal diaphragm separating the mercury and the gas. This was a one-shot system since the reservoir could not be refilled. The diaphragm was supposed to turn itself inside-out as the mercury was used up. However, the diaphragm often ruptured and also unfolded in such a way as to catch quantities of mercury in small pockets so that all of the mercury was not available for use. Flexible elastomeric diaphragms were found to be subject to most of the same disadvantages.

Further, because such regulators possess a small but finite leak rate and because gas must be vented to reduce system pressure, large amounts of gas would be consumed over the long hours) required life of an ion thrustor feed system. Rough calculation indicates that the high pressure gas bottle required would be larger than the mercury reservoir itself.

It is, therefore, a primary object of the present invention to provide a fluid feed system which is not subject to the above disadvantages inherent in previous feed systems.

It is a further object of the invention to provide a fluid feed system which does not require a bulky gas storage bottle or a mechanical regulator.

It is another object of the invention to provide a fluid feed system which does not require the venting of gas to reduce pressure on the fluid.

It is still a further object of the invention to provide a fluid feed system in which the only moving part is a slow moving piston which rides on Teflon bearings.

It is another object of the invention to provide a fluid feed system having a fast time response.

It is another object of the invention to provide a fluid ice feed system which requires little or no operating power.

It is another object of the invention to provide a liquid mercury feed system which provides smooth, high pressure, mercury feed at low flow rates.

It is another object of the invention to provide a propulsion system comprising a liquid mercury cathode and liquid mercury storage and flow control means.

It is another object of the invention to provide a fluid feed system in which essentially all of the fluid is available for use.

These objects are accomplished according to the present invention as follows:

The system of this invention employs a mercury storage reservoir and a piston which controls the mercury pressure in the reservoir. This invention includes the following three embodiments. In two of these embodiments the piston in the reservoir is connected to a pressurizer which comprises another piston-cylnder arrangement. The pressurizer includes liquid in equilibrium with its vapor and a heater for controlling the temperature of the pressurizer liquid and thus the pressure applied to the piston in the reservoir. In these embodiments only a small amount of liquid is employed and it is thermally isolated from the remainder of the structure, so that fast time response can be achieved (in addition by ependiture of low amounts of power). When operated in series with a downstream flow impedance, these two embodiments provide a continuously adjustable mercury flow rate which depends only on an input power to the heater. Mercury pressures from 0 to at least 200 p.s.i. are obtainable at a power input of about 10 watts. This system is applicable for use with any liquid or gas and is operable over a temperature range limited only by the structural materials. In the third embodiment a push rod is connected to the piston in the reservoir and pressure is applied to the push rod by means of constant force springs. This embodiment maintains a constant pressure on the fluid in the reservoir with no input power.

These and other objects and advantages of the present invention will be more fully understood by reference to the following detailed description when read in conjunction with the attached drawings in which:

FIG. 1 is a cross-sectional partly schematic view through one preferred embodiment of the present invention.

FIG. 2 is a cross-sectional view through another embodiment of the present invention, and

FIG. 3 is a cross-sectional view through another embodiment of the invention.

Referring now to the drawings, FIG. 1 shows a liquid mercury reservoir 2 adapted to contain a quantity of mercury in a chamber 4 therein. In the preferred embodiment, the mercury in chamber 4 is fed under pressure through a conduit 5 to a thrustor 7 (such as a Kaufman engine). Other elements of such a system, such as valves, a vaporizer and an isolator, form no part of this invention and need not be described in detail herein. However, in one embodiment to be described below the control for the propellant feed includes a closed loop in which changes in the ion beam provide the control signal to vary the propellant feed rate. It is noted that this system is applicable to other liquids than mercury and also to fluids and for other applications than for feeding propellant to ion engines.

Attached to the reservoir 2 is a pressurizer section 6 which contains a piston and cylinder arrangement comprising a stationary cylinder 10 inside of which is mounted a piston 8 comprising a piston head 9 attached to a bellows 11. A liquid is contained in the chamber 12 between the piston 8 and the cylinder 10. In the preferred embodiment, this liquid is water in equilibrium with its saturated vapor. The vapor pressure of the liquid is a unique function of the temperature of the cylinder 10. The temperature of the liquid in the chamber 12 is controlled by means of a heater 14 which in the embodiment shown in FIG. 1 comprises a heater filament helically wound upon and in contact with the outer surface of the cylinder 10. As the volume of the cylinder 12 increases due to motion of the piston 8, the liquid in the chamber 12 evaporates so as to maintain the saturated vapor condition. The pressurizer 6 and the reservoir 2 are thermally isolated from each other and the force from the liquid in the chamber 12 is transmitted from the pressurizer 6 to the reservoir 2 through a push rod 16 of low thermal conductivity material. A stainless steel shield 13 aids in insulating the pressurizer 6 from the reservoir 2. Thermal isolation is desired so that the temperature of the relatively low mass of the pressurizer may be rapidly changed by small amounts of input power. With tight thermal coupling between the pressurizer 6 and the reservoir 2, it would be necessary to change the temperature of both the pressurizer 6 and the mercury reservoir 2 which could contain several hundred pounds of mercury. In such a case, this system would have intolerably slow response.

The mercury in the chamber 4 is contained by means of a seal 18 between the side of the reservoir 2 and a piston 20. The side of the reservoir 2 and the piston 20 may be considered another piston-cylinder configuration similar to the piston-cylinder configuration in the pressurizer 6. The seal 18 is preferably of the rolling diaphragm type to reduce friction and contamination. If an all-metal system was required for high temperature or high pressure operation, the seal 18 can equally well be made with a metal bellows which allows a long stroke. The piston 20 is provided with a cylindrical skirt 22 to prevent the diaphragm 18 from bulging as the mercury is expelled and to insure that all of the mercury is available for use in the system.

In actual operation the embodiment shown in FIG. 1 has been used to supply liquid mercury flow rates from to 1 cc./hr. to a liquid mercury cathode while it was operating in an ion thrustor and to supply liquid mercury at constant pressure (about 7 p.s.i.) to a vaporizer operating as part of a vapor feed system for an ion thrustor using an oxide cathode. Closed loop operation using the ion beam as the input signal has been demonstrated to be useful with a liquid mercury thrustor. This closed loop operation is schematically shown in FIG. 1.

FIG. 1 shows an ion thrustor 24 connected to the chamber 4 by means of the conduit 5. The thrustor 24 can be any of the known types of ion thrustors, such as the Kaufman engine, and can employ, for example, a liquid mercury cathode or an thermionic cathode. In the case of the thermionic cathode, the mercury is supplied from the reservoir under constant pressure; this type of propellant control according to this invention is described below with reference to FIG. 2.

In the embodiment shown in FIG. 1 a control circuit is illustrated for use with a liquid metal cathode in which application it is desirous to vary the pressure on the mercury in the reservoir in order to vary the flow of propellant to the thrustor. The beam current is passed through a fixed resistor 26 to develop a voltage. This voltage is compared to a reference voltage 27 by means of a differential amplifier 28 and the difference signal is amplified by a power amplifier 29 such as a magnetic amplifier and a silicon controlled rectifier and used by a power controller 30 to adjust and control the power going to the heater 14 of the pressurizer 6 which ultimately controls the mercury flow and hence the ion beam current. In other applications of this invention the control signal to the power controller 30 can originate from a fluid flow measuring device positioned in the outlet conduit 5.

FIG. 2 shows another embodiment of the invention. A mercury reservoir 40 contains the mercury propellant in a chamber 42 defined by cylindrical walls 44. The mercury is forced out of the chamber 42 and through conduit 46 and into an ion thrustor 48, by pressure applied to a piston 50. The piston 50 is connected by means of a push rod 52 to another piston 54 in a pressurizer 56. The pistons 50 and 54 are provided with cylindrical skirts 58 and 60, respectively, for the same purpose as they are used in FIG. 1. Water and water vapor are contained in the chamber 62 in the pressurizer 56. Rolling diaphragms 64 and 66 used with each of the pistons 50 and 54 respectively.

That embodiment of the invention in which the mercury is maintained under constant pressure is shown for use in this embodiment. A thermistor 68 connected to the wall of the pressurizer 56 is coupled to a power controller 70 in the power supply circuit for the heater 72.

The pressurizer 56 is equivalent to the reservoir 40 except that the mercury is replaced by a small quantity of volatile liquid-in this case, water. The vapor pressure of the liquid in the pressurizer is directly related to the temperature, and thus the force on the mercury may be controlled by controlling pressurizer temperature. By thermally isolating the pressurizer from the reservoir, it is possible to effect rapid pressure changes at low input power levels, since the relatively small pressurizer mass is thermally decoupled from the large mercury mass in the resservoir. With the particular design used here, the pressurizer operating temperature of 95 C. is maintained with 5 w. of input power.

This embodiment has several advantages: (1) it has low operating power, (2) there are no Hooks law forces involved, the mercury pressure is directly related to the temperature of the pressurizer, therefore no pressure sensor is required, and a simple temperature regulator maintains constant (or controlled) mercury pressure.

Greater than 95% of the mercury can be expelled by the present invention and the system is readily cycled and refilled. This is not possible with metal diaphragm systems which crease or wrinkle as the mercury is expelled.

The embodiment shown in FIG. 3 employs a reservoir containing a chamber 82 for mercury, which chamber is defined by the walls 84, a piston 86, and the end wall 88 containing an orifice 90 for providing liquid communication to a thrustor 92 via a conduit 94. The chamber is sealed by means of a rolling diaphragm 96 connected between the walls 84 and the piston 86.

In this embodiment pressure is applied to the piston 86 (and consequently the mercury in chamber 82) via a push rod 98. Attached to opposite sides of the extreme end 100 of the push rod '98 are similar constant force springs 102 and 104. The springs 102 and 104 are mounted on a plate 106.

What is claimed is:

1. Apparatus for controllably discharging a fluid material at a constant pressure comprising:

a first hollow cylinder having first and second end walls defining a closed reservoir for said fluid material,

an outlet conduit connected to the first end wall for providing fluid communication with said fluid material,

said second end wall comprising a first movable piston in sealing engagement with the walls of said cylinder for applying pressure to said fluid material, and

means for applying pressure to said first movable piston and wherein said means for applying pressure solely comprises constant force spring means.

2. Apparatus for controlling the flow of liquid mercury from a reservoir to an ion thrustor comprising:

a reservoir adapted to contain a quantity of mercury comprising a piston slideably mounted in a cylinder in sealing engagement with the walls of the cylinder,

an ion thrustor,

conduit means connected between said reservoir and said thrustor for providing fluid communication therebetween, and

constant force spring means connected to said piston for applying a substantially constant force thereto.

References Cited UNITED STATES PATENTS 6 3,052,088 9/1962 Davis et a1 60202 3,097,483 7/1963 Bixson et a1. 60-39.48 XR 3,105,352 10/1963 Corbett 60-39.48 XR 3,159,967 12/1964 Webb 60-202 3,180,089 4/ 1965 Dodge 60--39.,48 3,256,686 6/ 1966 Lindberg 60-25 3,286,887 11/1966 Sudholm 222386 US. Cl. X.R. 

